The present invention relates to an image-forming apparatus using an electrophotographic system, such as a copying machine, a printer, etc., and an intermediate transfer material which is used for the image-forming apparatus, and particularly to an intermediate transfer material onto which a toner image formed in a latent image-carrier is primary-transferred and also to an image-forming apparatus obtaining a reproduced image by transferring the toner image transferred to the intermediate transfer material onto a recording medium such as a paper, etc.
In an image-forming apparatus using an electrophotographic system, a uniform electrostatic charge is formed on an electrostatic latent image-carrier which is a photoconductive photoreceptor formed by an inorganic or organic material and after forming an electrostatic latent image on the latent image-carrier by a laser light, etc., of a modulated image signal, the electrostatic latent image is developed with a charged toner to form a visualized toner image. Also, by electrostatically transferring the toner image onto a recording medium such as a recording paper, etc., directly or via an intermediate transfer material to obtained a desired reproduced image.
In particular, an image-forming apparatus employing a system wherein a toner image formed on the above-descried latent image-carrier is primary-transferred onto an intermediate transfer material and further the toner image on the intermediate transfer material is secondary-transferred onto a recording paper is known as disclosed in Japanese Patent Laid-Open No. 206567/1987, etc.
As the intermediate transfer material used for the image-forming material, the electrically conductive endless belts of thermoplastic resins such as polycarbonate resins (Japanese Patent Laid-Open No. 95521/1994), PVDE (polyvinylidene fluoride) (Japanese Patent Laid Open Nos. 200904/1993 and 228335/1994), polyalkylene phthalates (Japanese Patent Laid Open No. 149081/1994), a blended material of PC(polycarbonate)/PAT(polyalkylene terephthalate) (Japanese Patent Laid Open No. 149083/1994), a blended material of ETFE (ethylene-tetrafluoroethylene copolymer)/PC, ETFE/PAT, or PC/PAT (Japanese Patent Laid Open No. 149079/1994), etc., are proposed.
However, because the electrically conductive material of the thermoplastic resin such as a polycarbonate resin, PVDF (polyvinylidene fluoride), etc., is inferior in the mechanical characteristics (Young""s modulus 24,000 kg/cm2 or lower), the deformation of the belt to the stress at driving is large, whereby transferred images having a high quality are not stably obtained. Also, because cracks occur from the end portion of the belt at driving, the belt life is short.
As a material excellent in the mechanical characteristics, there is a polyimide resin and for example, in Japanese Patent Laid Open No. 311263/1988 (U.S. Pat. No. 2,560,727), a polyimide seamless belt having dispersed therein carbon black is proposed.
In a general molding method of the above-described polyimide belt, a film-forming base liquid which is a polyamide solution having dispersed therein an electrically conductive agent is poured in a cylindrical mold, and by rotating the cylindrical mold at a rotation number of from 500 to 2000 rpm while heating to a temperature of, for example, from 100 to 200xc2x0 C., a film is formed by a centrifugal molding method. Then, the film obtained is drawn from the mold in a semi-cured state and put over an iron core, and is polyimidated (ring-closing of polyamidic acid) at a high temperature of at least 300xc2x0 C. to carry out curing the belt.
Also, there is a method of casting the film-forming base liquid on a metal sheet at a uniform thickness, heating the case film at a temperature of from 100 to 200xc2x0 C. as described above to remove the greater part of the solvent, thereafter, raising the temperature stepwise to a high temperature of at least 300xc2x0 C. to form a polyimide film, and adhering both the ends of the film with an adhesive, etc., to form a endless belt.
In the case of using an intermediate transfer material formed with a thermoplastic resin such as polycarbonate having dispersed therein carbon black, an ethylene-tetrafluoroethylene copolymer having dispersed therein carbon black, etc.; or a polyimide resin having dispersed therein carbon black as in the prior art described above under a low-temperature low-humidity environment of 10xc2x0 C., 15% RH, when after continuously transferring toner images onto 3,000 or more transfer papers having a width shorter than the width of he intermediate transfer material, such as post cards (10 cm in width), a half tone (magenta 30%) image is transferred, the white spots are formed at the paper traveling portion as shown in FIG. 5. The occurrence of the e-white spots is caused by that because directly after the secondary transfer, the surface of the belt is charged to a plus side and belt side of the transfer paper is charged to a minus side as shown in FIG. 6, a releasing discharge occurs between the belt and the paper, whereby the surface resistivity of the paper traveling portion of the intermediate transfer material is lowered than those of the peripheral portions thereof and thus the transferring efficiency of the paper traveling portion is lowered than the transferring portion of the peripheral portions.
The present invention can solve the above-described problems in the techniques in prior art and also provide an intermediate transfer material and an image-forming apparatus using it capable of providing images of high image quality without lowering the surface resistivity of a paper traveling portion than those of the peripheral portions thereof and without causing image-quality defects such as white spots, etc., even when toner images are continuously transferred (transferring voltage is applied to the intermediate transfer material) to 3,000 or more transfer papers having a width shorter than the width of the intermediate transfer material, such as post cards, etc., under a low-temperature and low-humidity environment of 10xc2x0 C. and 15% RH.
That is, according to an aspect of this invention, there is provided an intermediate transfer material which is disposed between a latent image-carrier and a recording medium and is primary-transferred thereon a toner image formed on the latent image-carrier, wherein the intermediate transfer material is formed by a polyimide resin having dispersed therein an electrically conductive agent and having a glass transition temperature of 245xc2x0 C. or lower.
In the above-described intermediate transfer material, it is preferred that the above-described polyimide resin is a polybiphenyltetracarboxylic acid-base polyimide resin. Also, the surface resistivity of the intermediate transfer material is preferably in the range of from 1010 to 1014 xcexa9/xe2x96xa1.
Also, according to another aspect of this invention, there is provided an image-forming apparatus having a latent image-carrier forming thereon an electrostatic latent image according to an image information, a developing apparatus for visualizing the electrostatic latent image formed on the latent image-carrier as a toner image, an intermediate transfer material primary-transferred thereon the toner image carried on the latent image-carrier, and a secondary transferring apparatus for secondary-transferring the toner image on the intermediate transfer material onto a recording medium, wherein as the intermediate transfer material, the above-described intermediate transfer material is used.
The intermediate transfer material of this invention is formed by a polyimide resin having a glass transition temperature of 245xc2x0 C. or lower. The polyimide resin is obtained by a dehydrocondensation reaction of a tetracarboxylic acid dianhydride or the derivative thereof and a diamine. The dehydrocondensation reaction is finished in the temperature region of at least 330xc2x0 C. For example, the glass transition point of a polybiphenyltetracarboxylic acid-base polyimide resin obtained by finishing the dehydrocondensation reaction carried out by heating to a temperature of 330xc2x0 C. for 20 minutes was 260xc2x0 C. It has been discovered that when the dehydrocondensation reaction is carried out at a temperature of 300xc2x0 C. or lower, a polyimide resin having a glass transition point of 245xc2x0 C. or lower is obtained and by using the polyimide resin for an intermediate transfer material, the lowering amount of the surface resistivity of the paper traveling portion is reduced, and the invention has been accomplished.
The reason that the glass transition point shifts to a low-temperature side is considered to be that the dehydrocondensation reaction of the above-described tetracarboxylic acid dianhydride and a diamine is not completed and by the existence of a part of the unreacted tetracarboxylic acid dianhydride or diamine, the resistance lower at the secondary transferring portion is reduced.